Accelerometer



May 19, 1942. J. A'. BUCHANAN AccjELERoMETER Filed'Jan. 2, 1941 JANE rATTORNEY Patented May 19, l v

I UNITED@ applicati@ 'January 2,1941, serial Namaste l is claims. (on rrr-311)" l (Granted under. ne t of March '3Q lssa, as mended Avril 30, 1928: amour-151) My invention relates to new and'usefulimprovements in accelerometers, as well v`as the auxiliary equipment used in connection there- I with, and also relates to a new combination of such auxiliary equipment with saidaccelerometers. w

The particular accelerometer towhich this invention relates is useful generallyin the eldof accelerometry as applied to structural mechan' ics, and may be used to determine the riding or impact producing qualities of automobiles, railway coaches or the like. However, this `instrument was designed primarily for use in the iield of accelerometry as applied to a'eionautics. Ac'- celeroxneter records of an aircraft have, been used forjthe purpose of observing and obtaining the dynamic forces acting thereon, as well-as theload factor while in ight, these factors being valuable to the designer since they indicate the magnitude and duration of the stresses to which the aircraft structure is subjected, These records may also be Aused to give an indication of the pilots ability, since in stunt ying and in landing )the acceleration along any" particular 1 axis indicates the ability oi the pilot to execute vthe particular stunt or landing, excessive accel# eration being generally undesirable. It will later be apparent that my invention is not only useful in the same orfsimilar manner as are the prior art aocelerometers, but because of its ruggedness and simplicity of design it has and will later acquire additional uses to which the fprior art accelerometers are not particularly adapted.

Accelerometers are broadly divisible into two general types or classes. namely, (1)' those in which the disturbance or lack of disturbance'of a predetermined force system is the criterion to establish whether a given magnitude of acceleration is or is not attained, and (2) those in which the amplitude of displacement o! an-acceleration-sensitive mass is the criterion used to establish the value of the maximum aoceleration'imposed.

The accelerometer National Advisory Committee for Aeronautics lwould fall within the latter class `and consists primarily or' a cantilever spring with meansy for recording thedeilections of its free endon the used and designed by the f surface of a moving drum. 'I'his type and the seismographic type rely upon recordings of the vibrations on a moving drum or lm, and in order to obtain the actual value of acceleration the operator is required to convert. at some later time, the accelerometer indications into acceler- :mageiwdc My inventionhmo're specifically relateszto acelf 'eroineters 'o i fthe, ilrst1type`, og, cla coelaccelerometer -of vmy invention over v`these prior l structures isv that the jpil'otv ,ory lo pealjxir isggiven ferent"tensior'i'.y f

`l`Orieffthey objects oilily venti vide 'ari I acceleror'neter which "is and adapted foruse vby Another object `is to producen rugg capable-of "withstai'din considerabl rough y farfldlirig without physica de angement repeated'` audible signal" when 'the sensitivf ele;

ment of a classifyingl ac'celertn'neter` from ,its normal position.

- Still anotherf'object roffiiiy 1n'veiitic'ixi is to vide 'a rlclassi'fying' `aecelerometer in which i particular attained "f acceleration i M'be classi# ned :according mythe Spit-eh for ctn distinctive characteristicl of thelp'roduced Afurther objectisto provide a celeroneter wherein*:the'moveinent"offthefsensiltive elementv is,v limited :to a 1 straight lline `1 path" fin one direction ,"onlyifso that-f`ayffforceslprdded by the"accelerationofi the body ito'l* wliihf" the in ation values. One particular advantage of the 5 strument is attachedthat do not have a comporpreueterm 'ed nent in the direction of the straight line movementofsaidelementwillhanoeifectuponthe operation of said accelerometer.

Accelerometry is an art as well as a science.v

and acceleration determinations may be in error because of faults of manipulation as well as by faults of design and theory. 'll'he new and useful features of my invention permit more accurate and reliable determinations to be made and lessen the degree of skill -required by operating personnel.

overtheentirerangeofadjushnentprovidedby 'aresimilarintbisrespecttothegraduationsof With the foregoing and other objects in view, x

the invention consists in the construction, combination and arrangement of parts hereinafter described and illustrated in the drawings. in which:

Fig. l is an elevational view of a multiple element instrument, the left-hand element being in longitudinal cross-section;

Fis. 2 is a wiring diagram for a control circuit incorporating an electrically operated interrupter; and

Fig. 3 shows a wiring diagram for a plurality of the control circuits shown in Fig. 2, in combination with the make" or break" contacts of a plurality of classifying accelerometer elements.

Referring to the drawings, each accelerometer unit consists of. a-base l which supports the cylindrical body portion Il of each instrument. The interior of each cylindrical body is provided with a suitable insulating material i2, preferably a hard material which has a low coemcient of friction. A compression spring I3 and an acceleration sensitive element I4,are placed within the cylindrical body, the spring being centered between the boss on the element I4 and the boss 28 on the base Ill. The boss 28 is also insulated' from the base I0 by means of the insulating insert 2l. Polished lugs 24, or other suitable anti-'friction guide means, are provided on the sensitive element vI4 so as to provide a full free sliding fit between the sensitive element 4tliicl the substantially frictionless insulated wall 'I'he body Il is' threaded at its open end I5 to engage the internally threaded cap I6. This cap I8 is provided with aninternal insulating bo'ss.

or stop Il which bears against the sensitive element i4. The sensitive element I4 has a stem i8 secured to one end thereof which projects outwardly through the open end of body Il, through the bore provided in the stop Il (constructed of suitable insulating material) and l on through the cap I6. A contact button lI9 and locking means 22 are secured to the outwardly projecting end of the stem I8. The cap I6 is provided with a conductive shoulder 20 which is threaded throughout a portion of its entire length to accommodate a cover 23. The end of the shoulder 20 should be specially surfaced and polished so that a good, clean, low resistance contact will be made with the under side of the contact button I9 when the sensitive element is displaced a critical amount.

After the instrument is assembled but before 'cap 23 is screwedin place, the clearance 2i between the button i9 and shoulder 20 is adjusted to an amount which determines the maximum displacement of the sensitive element. T he cap I6 is also adjustable on the body portion il, providing thereby a means for adjusting the spring compression and the force necessary to move the sensitive element against the action of this spring. The maximum distance determined by the adjustment of the contacts remains constant a'micrometer screw. 'Ihe bo il on the hase Il provides a convenient mounting for a detachable plug connection so that conductor wires may be led o3 from the base il which is common to alll elements, and the respective base bosses 26 within each of the elements.

InthecircuitdagramshowninFigZthe 'accelerometer contacts are represented at 4l. Both make and break" contacts are illustrated although a single set of either may be used, as will presently be described. The make and break contacts lead to the selector switch 4| and thence through the contacts of the main switch 42 to a suitable source of power represented by the battery 43. The common or grounded side of the contacts lead to an audible signalling device represented diagrammatically by the headset 44, and thence to the power source 42 through either of two circuits dependent upon the position of the second selector switch 45.

In one position of the switch 45, i. e., when it is rotated approximately 60 degrees counterclockwise from the position shown in Fig. 2. the signal circuit is carried through an interrupting device represented by the make and break contact portion 41 of an electro-mechanical vibrator l 46. When contacts 4l are closed a continuous audible signal is obtained in the headset 44, the pitch of which is dependent upon the frequency of current interruption at 41. The interrupter circuit has its own source of power 4U and control switch 49. A condenser 50 is provided across the breaker points to reduce arcing at the contacts and produce a clear audible signal in the headset 44. l

In the alternate position of the switch 45, when it is rotated approximately 60 degrees clockwise from the position shown in Fig. 2, the conductors and the switch 4l form a closed conductive path to the headset 44, shunting the contacts 41 of the an audible click is detected in the headset 44,

once for each make or break of the accelerorneter contacts 40. In the case of periodically changing acceleration, the magnitude of which exceeds the predetermined setting of the instrument, the frequency of the clicks or the pitch of the audible note (where the variations are suiilciently high to produce a continuous note) would indicate the period of the changing accelerations.

The device could thus be used in the determination of the period of a changing acceleration which periodically exceeds the predetermined setting of the instrument. This period of the changing acceleration could be more accurately determined merely by the addition of a suitable recording device connected in series with the headset 44. The main disadvantage of this type ofcontrol for obtaining sustained values of acceleration is that the operator may miss the audible click in the headset and would otherwise be unaware of theattainment of the preset acceleration.

Fig. 3 shows the circuit connection for two control elements operating in combination with a single headset. A separate interruptor is provided ior each element having a period or frequency oi' interruption different from that o! the interruptor provided for the other element. Thev same reference numerals as used in Fig. 2 indicate corresponding parts, the primed numerals indicating duplicated corresponding parts.

It is to be understood that while I have Ashown 'an electro-mechanical meansior producing an audible signal, I do not intend to be limited to such means alone. `My device has been used in a combination where an oscillating vacuum tube the scale 28. Cap I6 through the insulating boss II- bears against the lsensitive element I`4 which, accordingly, compresses the spring I3, causing it to exert aforceF against the sensitive element I4. When the element I4 is subjected to dynamic conditions which impose a component a of acceleration along its' `axis towards the base I0, such component a multiplied by the mass m ofthe -sensitive element I4 causes it to exert a force ma in a direction opposite to the applied force F. As long as force ma doesnot exceed F no -relative movement takes place between the contacts i9 and 20. However, if the imposed acceleration is such that force ma exceeds spring torce F,

then element I4 will move relatively toward base' I0, causing gap 2l to close, making electrical contact between button ,I9 and shoulder' 20. It should be noted that the unit is mounted so that the axis of the sensitive element is coincident with or parallel to the axis along which it is desired to determine the acceleration.

I iind it convenient and desirable to calibratemy accelerometer in gravity or g units of acceleration. The scale is absolute, i. e., zero reading indicates no acceleration whatever, and the nor- 'mal acceleration of the earths gravitational iield thus corresponds to one g. It will thus be/seen that my accelerometer automatically includes such component of gravity as may be' eiective along its axis at any instant. When used in an airplane in ilight,the resultant load acting on the airplane in the direction of the axis or the instrument is directly determined by multiplying the indicated accelerations (in a units) by the total weight of the plane. This is the term irequently dei'lried as wing load by aircraft' designers. A

It will be understood that while I have shown 4in Fig. r1 an accelerometer which employs a Pair fr\make contacts, break contacts could also be provided, for example, between the insulating stop l1 and the sensitive element I4, and the breaking" of sucii contacts could also be employed to control the audible signal circuit.

When plural interrupters are being used with, break contacts, as-shown in Fig. 3, a signal of a characteristic determined by the multiple frequencies of the interrupters 41 and 41' will be continuously audible in the headset 44 as long as both break contacts 4B are closed, or until the acceleration to/ which one of the sensitive elements I4 is set to'respond has been exceeded, at which time the break contact for the element would openand the characteristic of lthe audible signal would be changed.` The `pitch of the signal would now be determined bythe frelquency of the remaining interrupter 41', and the change in the pitch of the signal would be detected in the headset 44 to indicate the acceleration. Any further increase in acceleration would cause the remaining break contacts 40' to open, provided the-acceleration exceeds that for which the second element has been set to respond, and

no signal would now be heard in the headset 44. The break contact could be used to control the energization of. the coil ofa relay having normally closed contacts. The contacts of the relay 4would control thev audibley signal circuit so that the signalling device would-be energized only when thebreak contacts were opened, and the device could thus be used to produce an audible signal only after the attained acceleration was sumcient to break the contacts. v

When the "make contacts are used, which is the preferred form of my invention, the switch 4i would be rotated'to its counterclockwiseposition, the main switches 40 and 49 would also be closed. and switch 45 set in its" counter-clockwise position to Ainclude the' interrupter contacts 4l.l

It the imposed acceleration is insumcient toj close the "make contacts I9 and 20, there will be no' audible signal in the headset, and the acceleration indicated on scale 28 has not been attained.' When the imposed acceleration is suilicient to sitio vibrations.

close the make contacts I9 and 20, an audible signal or note will be heard in headset 44, and will be sustained so long as such imposed acceleration exceeds' the predetermined'setting indi cated at scale 28.

If two elements are used as indicated in Fig. 3, the first audible signal will indicate that the attained accelerationA is between the limits set on the two elements, vbut kif the second distinctive signal is heard the operator will know that the attained acceleration exceeds the predetermined value set on the second element. Naturally, the maximum and minimum values of the particular magnitude of acceleration can beadjusted merely by adjusting the predetermined level for each element. f

.The break contacts are not ordinarily as desirable as are the make contactsbecause when parasitic vibrations are present (as is frequently the case), the vibrations transmitted to the sensitive element I4 may cause a premature separation of the break contacts. producing a false interpretation or measurement of the basic acceleration of the apparatus under observation, Many of the prior art devices which operate on because of the contact .chatter and false recordings produced by a break resulting from para- In the present invention the spacing of the contacts I9 and 20 may be adjusted by means of the nut 22 to eliminate the effect of these parasitic vibrations transmitted to the element I4. Once the adjustment is made, the gap .2| will remain constant for all settings of thevinstrument. The vibrations usually `encountered require but a relatively Vsmall separation between the contacts i9` and 20 and no significant loss in accuracy vwill ensue.

It is to be understood that my acceierometer elements may be employed either singly, as indicated inv Fig. 2, or in multiple, as indicated in i force required to move the inertia member .against the -action cf said spring maybe readily computed inunitsf'cf "acceleration fand frequency sensitive me for Ndetecting, the critical movement of i' saidinertia member. g

The inventionderied in claim 8 characterlythefactthat the last named means incl es an'electromagnetic means for producing asu'stained audible signal of a pitch equivalent t he frequency. .of the'current variations in the cijr "whichit is connected, a circuit interrup an electrical powery supply source and m y'ns fory connecting the signal producing means to ai 'ipowejsupply source in series with the sa tacts andthe circuit interrupter whereby ,y ined"audible signal of a pitch equivalent 'to' the'freq'uency of thecircuit interrupter will be produced in said means when the contacts are 107;` :In a1 device lior indicating the attainment rents flowin. said conductive means upon the opofajpredeterminedjacceleration a circuit closer ofthe actuating current, a supply source of elec-"s tric current, conductive means,connectingAsaidl u '2: "1.

` eration of said circuitccntrlling means whereby `-corxiprising two normally spaced contacts, means the attained acceleration may 'be classified acfor adjustably setting the spacing between said cording to the frequency indications of the curcontacts, a movable inertia member, a guide suprent variations in .said conductive means. eport therefor having a base, yieldable means for 6. In combination, a multiple element classiforcing said inertial member to a rest position fying accelercmeter comprising a plurality ofA acfrom which it is movable in a direction against celeration sensitive elements; a plurality of electhe force of said yieldable means in response to tric contact means each of which is controlled the attainment of a predetermined forcing accelby one of the accelerometer elements, a plurality eration, the force of said yieldable means being of electrical supply sources each having a dis variable in accordance with the position of said l tinctive frequency, detecting means sensitively re.. member relative to said base, operative means sponsive to 'the several electric supply frequenconnecting said inertia member to one of said cies, connecting means between the said accelcontacts to move lsaid contact into conductive erometer controlled contacts, the supply fi'e- 35 engagement with said other contact upon movequencies and the'detecting means, whereby the ment of said member in the direction against characteristics of an limpressed or forcing accelsaid applied force, positioning means for holding eration maybe sensitively. determined according said member in its rest position against the .force to changes in=the characteristics of the frequenof said yieldable means, and means for ladjustcies transmitted tothe detecting means. 4o ing said positioning means relative to said base '7. An acceleration indicating device comprising which will maintain the set spacing between said a plurality of inertia actuated elements each movcontacts. able in response to a dierent predetermined 11. A vclassifying accelerometer comprising an magnitude of acceleration, a circuit'l controlling inertia responsive member, yieldable means forcmeans for each individual element operated by' 45 ing said member to a position from which it is the movement thereof, a multifrequency variable movable against the action of the said yieldable current supply source, separate' circuits connectsforcingV means through a critical distance when ing each controlling means to said supply source subjected to impressed accelerations having a so that upon movement of each element operating .magnitude greater than a predetermined minithe circuit controlling means the current variamum value, a circuit closurel having a pair of tions in each o f said circuits vwill be of different normally spaced and relatively movable contacts, frequencies and means for indicating the'irequen-A means responsive to the movement of said incy of the current varitions in each circuit, whereertia member through said critical distance for by through the use of a plurality of elements the effecting engagement of said contacts, means for attained acceleration maybe classified according obtaining an adjustment of said critical distance to the frequency indications. Y whereby the movements of said member in re- 8. A contact accelerometer comprising a base sponse in parasitic vibrations of relatively high member, a hollow cylindrical body mounted on frequency will be less than said critical distance,

, said base, a cylindrical inertia member supported means for adjusting the said yieldable means and guided` by said body member so as'to have without affecting the adjustment of the said crita critical axial movement relative thereto, a

ical distance to vary the said predetermined minspring forcing against said inertianm'embenaconimum value of acceleration response, a supplyvv tact carried by said inertia member and movable' source of electric current varying at an audible therewith, a contact carried by said body, the confrequency, and means connected to said supply tacts being positioned sothat they maybebrought 6r source through said contacts and actuated by into current conductive engagement upon the the current variations therein upon engagement critical axial movement of said inertia member, of said contacts for translatingsaid current varmeans for. obtaining a delicate precise adjustment iations into sensorially detectablev vibrations conof the spacing of said contacts to change the crittrolled by said current variations and having the .ical movement of said inertia member in order characteristics of an audible note.

that its movement in responseto parasitic vibra- 12. A classifying accelerometer comprising a tions will not aect the circuit controlled by said plurality of inertia actuated members, each of contacts, means for adjusting the force of said said members being adjustably set to respond to.

spring without changing the space between conlsuccessive predeterminedfincrements in magnitacts, said means being calibrated so that the 7 5 tucie of forcingacceleratiomjeach of said mem-l 

